1. Field of the Invention
This invention relates to silicon rubber compositions and more particularly, to liquid silicone rubber compositions which are especially useful as an insulating adhesive or potting material for various printed circuits. The composition has a low viscosity with good workability and exhibits high thermal conductivity when vulcanized.
2. Description of the Prior Art
Electrically insulating adhesives or potting materials for printed circuits and hybrid integrated circuits having transistors, integrated circuits and memory elements are generally required to dissipate heat generated during operations by thermal transmission. To this end, thermal conductive compositions have been used as such adhesives or insulating potting materials. Japanese Laid-open Patent Application No. 55-22891 describes one of such compositions, which contains polyorganosiloxane, aluminum oxide, beryllium oxide or magnesium oxide, and an organic peroxide. However, this composition is not fluid but is solid in nature. Thus, the composition cannot be employed as a potting agent.
Japanese Laid-open Patent Application No. 50-2349 describes a composition which comprises an addition-reactive mixture of a vinyl group-containing organopolysiloxane, organohydrogenpolysiloxane and a platinum compound, to which alumina is added. This composition is disadvantageous in that it is dark in color and has poor thermal conductivity because the composition is unlikely to be dispersed uniformly.
To overcome the above disadvantage, Japanese Laid-open Patent Application No. 58-219259 proposed an improved composition in which the alumina used is defined to have an average particle size of 2.0 to 10 micrometers and an oil absorption of not less than 15 ml/g. This composition has the drawback that it has high viscosity prior to vulcanization and the product obtained by vulcanization of the composition is unsatisfactory with respect to the thermal conductivity. Thus, satisfactory thermal conductive silicone compositions useful for the above purposes have not been obtained yet.